Ultra low-cost solid-state memory

ABSTRACT

A three-dimensional solid-state memory is formed from a plurality of bit lines, a plurality of layers, a plurality of tree structures and a plurality of plate lines. Bit lines extend in a first direction in a first plane. Each layer includes an array of memory cells, such as ferroelectric or hysteretic-resistor memory cells. Each tree structure corresponds to a bit line, has a trunk portion and at least one branch portion. The trunk portion of each tree structure extends from a corresponding bit line, and each tree structure corresponds to a plurality of layers. Each branch portion corresponds to at least one layer and extends from the trunk portion of a tree structure. Plate lines correspond to at least one layer and overlap the branch portion of each tree structure in at least one row of tree structures at a plurality of intersection regions.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part application ofcommonly assigned and co-pending U.S. patent application Ser. No.10/453,137 by Barry Stipe, filed Jun. 3, 2003, entitled “Ultra Low-CostSolid-State Memory,” and incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to solid-state memories. In particular,the present invention relates to a three-dimensional (3-D) arrangementof memory cells forming an ultra-low-cost solid-state memory.

2. Description of the Related Art

FIG. 1 shows a table setting forth estimated scaling limits, estimatedperformance characteristics and estimated costs for current andpotential solid-state memory technologies, as projected for the year2020. Revenue estimates for 2002 are given or are indicated as DEV fortechnologies that are still under development and RES for technologiesin the research stage. Important factors influencing the cost per bitfor the solid-state memories shown in FIG. 1 include the scalability tothe smallest dimensions, the number of bits per cell, and the cost ofthree-dimensional (3-D) integration.

The scaling limits indicated for each solid-state technology arespeculative and are based primarily on physical limits rather thancurrent technical challenges. The cost of processing a unit area ofsilicon has remained fairly constant over the years, and hashistorically been about ten times higher than the cost per unit area forlow-cost 3.5″ Hard Disk Drives (HDDs). It has been estimated that use of300 mm wafers will lower the cost per unit area by about 30%.Nevertheless, current desk-top HDDs are about 100 times cheaper per bitthan DRAM and FLASH memories because HDDs have an areal density that isabout ten times greater than DRAM or FLASH memories. For the memorytechnologies identified in FIG. 1 that are capable of low-cost 3-Dintegration, it is assumed that layers are added until the cost per unitarea increases by 60%, thereby providing a good trade-off between lowercost and manufacturability.

Four technologies may eventually reach a cost that is comparable to thatof HDDs through either multi-bit storage or 3-D integration, which aretwo characteristics that HDDs cannot practically possess. Two of thefour technologies, PROBE memories and MATRIX memories, are likely tohave performance characteristics that are inferior to HDDs. The othertwo technologies, Ovonic Universal Memory (OUM) and zero-transistorferroelectric memory (0T-FeRAM), are likely to have superior performanceto HDDs and are potential replacement technologies for HDDs. Even if ahigh-performance memory is twice as expensive as HDDs, it may still bewidely preferable because large amounts of DRAM (or other memory) wouldnot be required for buffering the processor.

The scaling limits and associated cost estimates for the various memorytechnologies shown in FIG. 1 are described below:

SRAM

Static Random Access Memory (SRAM) cell is formed by six MOSFETS, soscaling challenges are the same as for transistors and wires. The mostscalable MOSFET design is generally believed to be the double-gatetransistor. See, for example, J. Wang et al., “Does Source-to-DrainTunneling Limit the Ultimate Scaling of MOSFETs?” IEDM Tech. Digest(IEEE), p. 707 (2002). Because the gates must be insulated from thechannel and the insulation must be thicker than about 2 nm to preventexcessive gate tunneling current, the gates must be separated by atleast 4 nm plus the thickness of the channel. Roughly speaking, thechannel length must be at least as long as the gate-to-gate distance forthe transistor to properly turn off, even when a high-k dielectricinsulation is used. Consequently, the smallest workable transistor is onthe order of 5 to 6 nm in length.

Today, gate lengths are about 65 nm using lithography that is capable of130 nm half-pitch between wires so the smallest transistors would beappropriate for the 11 nm node in about 2020. See, for example,http://public.itrs.net. Extremely advanced lithography will be requiredfor 11 nm half-pitch node. The minimum half-pitch for Extreme-UV (EUV)lithography at a wavelength of 11 or 13 nm is given by F=k₁λ/NA, inwhich k₁ is a constant having a minimum value of about 0.25 using phaseshift masks, λ is the wavelength and NA is the numerical aperture havinga maximum value of about 0.55 for the reflective optics that are usedfor EUV lithography. See, for example, U.S. Pat. No. 5,815,310 to D. M.Williamson entitled “High Numerical Aperture Ring Field OpticalReduction System.” Although these particular parameters indicate thatthe lithography limit is about 5 nm half-pitch, it is unlikely thislimit will be reached.

If the more conservative parameter values are considered, i.e.,k₁=NA=0.4, then the limit is at the 11 nm node. If transistor gatelengths must be somewhat longer than 6 nm, memory density will not bevery adversely affected because the cell size is determined more by thewire pitch than by gate length.

The minimum cell size for SRAM is large at about 50 F², consequently,the maximum density for F=11 nm is about 0.1 Tb/in². It is expected thatSRAM will continue to be used in the future for applications in whichspeed is most important because SRAM is the fastest performing memorytype for both reading and writing.

DRAM

A Dynamic Random Access Memory (DRAM) cell is formed by a MOSFET and acapacitor. The voltage stored on the capacitor must be refreshed aboutevery 0.1 s due to leakage. DRAM memory has very serious scalingchallenges. See, for example, J. A. Mandelman et al., “Challenges andFuture Directions for the Scaling of Dynamic Random-Access Memory(DRAM),” IBM Journal of Research and Development, vol. 46, p. 187(2002). For example, one of the most serious scaling obstacles for DRAMmemory results from the adverse effects of radiation in which a singlealpha particle can create about 1 million minority carriers thatsometimes end up on the capacitor. For immunity from the effects ofradiation, the capacitor must hold more than 1 million electrons,corresponding to a capacitance of about 30 fF. See, for example, A. F.Tasch et al., “Memory Cell and Technology Issues for 64 and 256-MbitOne-Transistor Cell MOS DRAMs,” Proceedings of the IEEE, vol. 77, p. 374(1989).

In DRAM, reading the state of the capacitor is destructive, so the datamust be rewritten afterward. With conventional architecture, the stateof the capacitor is sensed by discharging the capacitor to a bit linehaving a capacitance that is much greater than 30 fF. Further reductionsin storage capacitance would lower the sense voltage to levels that arenot easily detectable. Because the capacitance cannot be readily scaled,the capacitor has presently taken the form of a cylinder extending deepinto the silicon wafer and having an aspect ratio of about 50 to 1. Anaspect ratio of this magnitude does not appear to be capable of beingincreased much further and soon capacitors will need to flare out underthe silicon surface taking the shape of, for example, a bottle. Also,high-k dielectrics, such as barium strontium titanate (BST), will beneeded for improving performance of the capacitor. Unfortunately, high-kdielectrics have a high leakage and, therefore, need a thickness that isthicker than that of the dielectric materials that are used today.Accordingly, the thickness of high-k dielectrics can add considerably tothe diameter of nanometer-scale capacitors. With such scaling obstacles,it seems unlikely that DRAM will scale to be smaller than about 30 nm.

HDDs

Historically, Hard Disk Drives (HDDs) have about ten times greater datadensity than DRAM or FLASH memories because there is little or no spacebetween bits and data tracks. Additionally, bit density along the trackis determined primarily by field gradient and head fly-height ratherthan by a minimum lithographic dimension. Only track density isdetermined by lithography. The areal density advantage of HDDs, however,is likely to decrease due to the superparamagnetic limit in whichscaling of magnetic grain size in the disk is no longer possible becausethermal energy k_(B)T begins to compete with the magnetic anisotropyenergy K_(u)V. For written data to be thermally stable for a period ofseveral years (at about 330 K), the minimum size of a magnetic grain islimited to approximately 8 nm.

Although materials exist having a minimum stable size of approximately 3nm, the coercivity of these materials is higher than the maximumattainable field that can be produced by a write head. About 10-20grains will be needed per bit to prevent excessive error correction fromreducing the data density because the grains are randomly oriented. See,for example, R. Wood, “Recording Technologies for Terabit per SquareInch Systems,” IEEE Transactions of Magnetics, vol. 38, p. 1711, 2002,and M. Mallary et al., “One Terabit per Square Inch PerpendicularRecording Conceptual Design,” IEEE Transactions of Magnetics, vol. 38,p. 1719, 2002.

Although it is generally accepted that the areal density limit forconventional recording is about 1 Tb/in², it may be possible to use arevolutionary technology, such as thermally-assisted recording in whichthe disk is heated to lower the coercivity of the media for writing.Nevertheless, there is a limit when thermal energy k_(B)T begins tocompete with the Zeeman energy 2H_(A)M_(S)V, in which H_(A) is theapplied field, so that the grains are not properly oriented duringwriting. This effect limits the grain size to about 4 nm, which is afactor of two smaller than the grain size used for conventionalrecording. Unfortunately, there is no known practical way to make ananometer-scale heat spot on the disk. Patterned media has also beenproposed as a way to surpass 1 Tb/in². An e-beam master is used to stampa pattern into the disk to form magnetic islands so that there can beonly 1 grain per bit. Unfortunately, e-beam lithography resolution islimited by secondary electrons exposing the resist, thereby making itcurrently impossible to surpass 1 Tb/in². See, for example, S. Yasin etal., “Comparison of MIBK/IPA and Water/IPA as PMMA Developers forElectron Beam Nanolithography,” Microelectronic Engineering, vol. 61-62,p. 745, 2002. FIG. 1 indicates the density limit for HDDs to be 1Tb/in², which may be reached as early as the year 2010.

FLASH

FLASH memory technology uses a single floating-gate transistor per cell.Typically, FLASH memory is used when an HDD is too bulky. FLASH memoryhas a fast read time, a relatively slow write time, a low data rate andlow endurance. The cost of FLASH memories, however, is rapidly droppingand is expected to be the fastest growing memory type over the next fewyears, especially for NAND and AND-type FLASH memory architectures. Forsmall capacities, the cost of FLASH memory is currently cheaper thanHDDs because HDDs cannot cost much less than $50 based on fixed costs.Today, FLASH memory prices are cut in half every year due to aggressivescaling and the recent introduction of two-bits-per-cell technology.Four-bits-per-cell technology is expected to be available within a fewyears.

Although multi-bit storage techniques reduce estimated costdramatically, multi-bit storage typically leads to lower performancebecause the read/write process is more complicated. The capability ofFLASH memory to store multiple-bits per cell is based on the ability ofthe floating gate to store a very large number of electrons, therebyvarying transistor conductance over many orders of magnitude. Therefore,FLASH memory has very fine granularity and low noise with today'stechnology.

FLASH memory, however, has very serious scaling challenges because thedielectric around the floating gate must be at least 8 nm thick toretain charge for ten years. See, for example, A. Fazio et al., “ETOXFlash Memory Technology: Scaling and Integration Challenges,” IntelTechnology Journal, vol. 6, p. 23, 2002. This thickness is four timesthicker than the thickness of the gate dielectric used in SRAM. Also,the voltage used for programming FLASH memories must be greater thanabout 8 volts, making it difficult to scale the peripheral transistorsthat are used to supply the programming voltage.

NOR FLASH memory is not believed to be scalable past the 65 nm node dueto problems with drain-induced barrier lowering during programming atthis length scale. See, A. Fazio et al., supra. Similarly, NAND FLASHmemory is projected to have very serious scaling challenges below 40 nmdue to interference between adjacent gates, particularly for multi-bitstorage. See, for example, J.-D. Lee et al., “Effects of Floating-GateInterference on NAND Flash Memory Cell Operation,” IEEE Electron DeviceLetters, vol. 23, p. 264, 2002.

Scaling projections for NAND FLASH memory, which are shown in FIG. 1,are based on the assumption that further improvements will scale NAND orNROM FLASH memory to about 30 nm half-pitch using four-bits-per-celltechnology. Below this size, the small number of electrons per bit, thesize of the high voltage circuits, and interference between chargestorage regions will likely cause obstacles too significant for furtherscaling.

Probe

Probe memory technology primarily refers to the “Millipede” concept fordata storage being pursued by IBM in which a 2-D array of siliconcantilevers having very sharp silicon tips are scanned over a thinpolymer film on a silicon substrate and heated for poking holes in thepolymer. See, for example, P. Vettiger et al., “TheMillipede—Nanotechnology Entering Data Storage,” IEEE Transactions ofNanotechnology, vol. 1, p. 39, 2002. Bits are detected by sensing thecooling of the cantilever when the tips dip down into the holes. Accesstimes are about as long as for an HDD because the entire chip must bemoved relative to the tip array to reach the desired memory address.Data rates are quite low compared to HDDs. That is, it will take a rowof 400 cantilevers in a 160,000 cantilever array operating at about 100KHz each to achieve a data rate of 4 MB/s. If this data rate can beachieved, PROBE memory would be competitive with FLASH and the 1″Microdrive.

Power dissipation, however, is very high for both reading and writingbecause micron-scale heaters are used at temperatures of up to 400 Cdissipating about 5 mW each. Consequently, a 4 MB/s data rate wouldrequire 2 W of dissipation, thereby making PROBE storage two times lessenergy efficient per bit than the Microdrive and at least 20 times lessefficient than FLASH memory. PROBE storage is inherently 2-D in natureand is not likely to be capable of multi-bit storage due to noise andother issues, although in theory there could be three layers of polymerwith different glass transition temperatures to vary the depth withapplied temperature and store 2 bits per indent.

So far, the estimated cost per unit area is uncertain, but is likely tobe at least as expensive as other solid-state memories because twosilicon wafers are used in a precise sandwich arrangement and asubstantial amount of peripheral control circuitry is needed. Alignmentand thermal drift are a major problem and it is likely that a number ofthermal sensors and compensation heaters will be needed to keep top andbottom wafers isothermal and to within one degree of each other. Tipwear and polymer durability are other major issues.

PROBE storage, however, has a major advantage in that bit size isdetermined by tip sharpness rather than by lithography. Also, becausethe polymer is amorphous, grain size limitations do not occur. In thatregard, IBM has demonstrated an areal density of 1 Tb/in² using silicontips. Improvements in tip technology might make it possible to improvethe density significantly. Local oxidation storage at >1 Tb/in² has beendemonstrated with a nanotube tip. See, for example, E. B. Cooper et al.,“Terabit-Per-Square-Inch Data Storage with the Atomic Force Microscope,”Applied Physics Letters, vol. 75, p. 3566, 1999. If a manufacturablemethod of forming ultra-sharp, durable tips can be developed, perhaps 10Tb/in² is possible. See for example, E. Yenilmez et al., “Wafer ScaleProduction of Carbon Nanotube Scanning Probe Tips for Atomic ForceMicroscopy,” Applied Physics Letters, vol. 80, p. 2225, 2002.

OUM

Another emerging memory technology is known as Ovonic Universal Memory(OUM). See, for example, M. Gill et al., “Ovonic Unified Memory—aHigh-Performance Nonvolatile Memory Technology for Stand-Alone Memoryand Embedded Applications,” ISSCC Tech. Digest (IEEE), p. 202, 2002. OUMuses one programmable resistor and one diode (or transistor) per cell.The high and low resistance states of a phase-change resistor (amorphousversus crystalline) is used for storing bits. OUM writing isaccomplished by passing high current through the resistor to bring thematerial to the crystallization temperature or melting temperature(about 400 to 600 C). Rapid cooling of the melted material results inthe amorphous (high resistance) phase. Writing the crystalline phaserequires a longer time for nucleation and growth to occur (about 50 ns)and results in about 100 times lower resistance than in the amorphousphase.

Intermediate values of resistance can be set by controlling the current(and, therefore, temperature) during the programming pulse, therebymaking multi-bit storage possible with OUM, but likely to be moredifficult than for FLASH memory because the phase-change resistorscannot be accessed directly like the transistors in a FLASH memory.Direct access is not possible when a diode is used to prevent multiplecurrent paths through the cells. A series diode effectively reduces thechange in resistance from a factor of 100 to only about a factor of two.FIG. 1 indicates that two-bits-per-cell technology will be possible withOUM.

OUM is scalable because the resistance is determined by the position ofthe amorphous-crystalline boundary and has atomic-scale granularity.Although the phase-change material must be heated to very hightemperature, the small programming volume results in reasonable powerdissipation. OUM has a scaling problem in that power per unit area andcurrent density scale inversely with size at constant peak temperaturebecause the temperature gradient scales inversely with size. It isexpected that current density will need to be in excess of 10⁷ A/cm² toheat up a volume that is 10 nm across to 600 C, even with excellentthermal isolation.

Nanoscale copper wires are known to have an electromigration time tofailure of a few years at this current density and will quickly bedestroyed at 10⁸ A/cm². See, for example, G. Steinlesberger et al.,“Copper Damascene Interconnects for the 65 nm Technology Node: A FirstLook at the Reliability Properties,” IEEE Interconnect TechnologyConference Proceedings, p. 265, 2002. Problems of electromigration canprobably be avoided by using interconnects having a tall aspect ratio,although local electromigration near the devices could still be asignificant problem.

Another issue that may be associated with OUM is the need for bulkytransistors for driving large current densities, even though ashort-channel length will help alleviate this potential problem. Theneed for large current density and a diode at each cell for preventingmultiple current paths when accessing a cell will make 3-D integrationof OUM quite difficult. Polycrystalline silicon diodes fail quickly atcurrent densities of about 10⁶ A/cm². See, for example, O.-H. Kim etal., “Effects of High-Current Pulses on Polycrystalline Silicon Diodewith N-Type Region Heavily Doped with both Boron and Phosphorus,”Journal of Applied Physics, vol. 53, p. 5359, 1982. In particular,polycrystalline silicon diodes are only reliable below current densitiesof about 10⁵ A/cm². See, for example, U.S. Pat. No. 6,429,449 to F.Gonzalez et al., entitled “Three-Dimensional Container Diode for Usewith Multi-State Material in a Non-Volatile Memory Cell”.

A diode surface area 100 times larger than the area of the resistorwould be required if polycrystalline silicon were used. Additionally, alarge number of processing steps would be required to make a tallcylindrically-shaped diode. See, for example, U.S. Pat. No. 6,429,449 toF. Gonzalez et al. Very tall diodes would mean very high aspect ratiosfor the diodes and for the vias between layers. Even if very large grainsize is achieved with a planar diode, a single grain boundary orintra-grain defect can cause a device to fail given the current densityneeded to write OUM memory.

Wafer bonding techniques used to make Silicon-On-Insulator (SOI) can beused to form diodes in multiple layers if single crystal silicon must beused. See, for example, K. W. Guarini et al., “Electrical Integrity ofState-of-the-Art 0.13 μm SOI CMOS Devices and Circuits Transferred forThree-Dimensional (3D) Integrated Circuit (IC) Fabrication,” IEDM Tech.Digest (IEEE), p. 943, 2002. To keep costs down, it is advantageous tobond a very thin layer of silicon while reusing the host wafer. Oneprocess that appears suitable for making 3-D ICs with single crystalsilicon is based on the ELTRAN method that has been developed by Canon.See, for example, K. Sakagushi et al., “Current Progress in EpitaxialLayer Transfer (ELTRAN),” IEICE. Trans. Electron., vol. E80C, p. 378,1997. According to the ELTRAN method, a host wafer is etched to form aporous layer having very small holes at the surface and large cavitiesmuch further down. Epitaxial silicon then bridges the holes to form anew, very high quality surface layer that may undergo the hightemperature (>600 C) processing that is needed to form diodes ortransistors.

Subsequent steps can be carried out at lower temperature (<600 C) toprevent damage to the 3-D chip. The epitaxial layer is bonded to the 3-Dchip and cleaved along the weak porous layer. Alternatively, theepitaxial layer is bonded to a transparent transfer wafer, cleaved, andthen transferred to the chip. Etching and chemical-mechanical polishing(CMP) is used for resurfacing the two cleaved planes and the host waferis reused. Low temperature processing, such as making phase-changeresistors, can be performed on the 3-D chip before the next siliconlayer is added. The advantage of OUM memory over other similar schemesbased on a field-programmable resistor is that current passes in onlyone direction through the resistor so a diode can be used instead of atransistor for access, thereby reducing the size of the cell and thenumber of processing steps for each silicon layer. Even though the costof single crystal silicon is high, 3-D integration should reduce costmore for OUM than for technologies that require a single crystal MOSFETin each cell.

Roughly estimated costs associated with OUM include about $5000 forprocessing a 300 mm wafer into chips, yielding up to 1000 dies of 70mm², each costing about $5. EUV lithography is expected to be expensiveat $40 per mask step. See, for example,http://www.sematech.org/public/resources/litho/coo/index.htm. Assumingfive masks per layer and three layers, $600 is added to the estimatedcost of the wafer. Today SOI wafers are very expensive at over $1000each with the cost projected to drop to $700 over the next few years. Ifthe cost can continue to drop to about $600, it is projected that threeadditional layers of silicon will cost about $1800 per 3-D wafer. Ifanother $600 is budgeted for additional processing steps, costs for themasks and costs for testing, the total cost increases 60 percent, butthe memory density increases by a factor of four, assuming the bottomlayer also has memory cells. According to FIG. 1, OUM may eventuallyreach an estimated cost that is close to that estimated for HDDs when(1) less expensive SOI techniques (by today's standards) can be used for3-D integration, (2) multiple bits can be stored per cell, and (3)lithography can be extended down to 10 nm.

MJT-MRAM and 3D-MRAM

Magnetic Random Access Memory (MRAM) uses one magnetic tunnel junctionand one diode (or MOSFET) per cell. The high and low resistance statesof a MTJ (i.e., parallel versus anti-parallel magnetic electrodes) areused for storing bits. See, for example, K. Inomata, “Present and Futureof Magnetic RAM Technology,” IEICE. Trans. Electron., vol. E84-C, p.740, 2001. Magnetic Tunnel Junction MRAM (MTJ-MRAM) writing isaccomplished by passing current through word and bit lines to create amagnetic field that is sufficiently strong to switch the “soft” or“free” magnetic electrode at the cross point of the word and bit lines.

It would be difficult to store more than one bit per cell in MRAM due tothe squareness of the MTJ hysteresis loop. One possibility forovercoming this difficulty would be to connect three MTJs in series witheach MTJ having a different threshold to store two bits. The complexityand cost of connecting three devices in series for storing twice as muchinformation needs further consideration. For that reason FIG. 1indicates that MRAM can have only one bit per cell.

A significant obstacle associated with MTJ-MRAM is that the currentdensity necessary for producing a write field scales poorly as the wiresare made smaller. The poor scaling is related to a necessary increase inthe coercivity of the soft electrode to avoid the superparamagnetismeffect. For example, to scale to the 40 nm node, a cube-shaped magneticbit needs an anisotropy energy of K_(u)=50 k_(B)T/V=3.5×10⁴ ergs/cm³.Assuming a magnetization of 1000 emu/cm³, the anisotropy field wouldneed to be H_(k)=2 K_(u)/M=70 Oe. Using the Stoner-Wohlfarth model ofmagnetic reversal, H_(k) can be taken to be approximately equal to thefield necessary for fast switching. For 40 nm×40 nm bit and word wires(at 45 angular degrees to the magnetic axis) to produce a magnetic fieldof 70 Oe 40 nm from the wire centers, the current density would need tobe at least j=(5/2^(1/2))H_(k)/d=6×10⁷ A/cm². As discussed above, copperwires will fail after a few years at only 1×10⁷ A/cm², so scalingMTJ-MRAM even to 40 nm will require large improvements in theelectromigration resistance of copper wires. Consequently, the cost ofMRAM will remain quite high in comparison to more scalable technologies.

MRAM does have one interesting advantage for low cost: high current doesnot need to pass through the cell because writing is accomplished withmagnetic fields. During a read operation, a diode may be needed forpreventing multiple current paths in the cross-point architecture, butthe diode can be made from thin film amorphous silicon. See, forexample, P. P. Freitas et al., “Spin Dependent Tunnel Junctions forMemory and Read-Head Applications,” IEEE Transactions of Magnetics, vol.36, p. 2796, 2000. Although a thin film amorphous silicon diode is muchcheaper than a single crystal silicon diode; the maximum current densitythrough amorphous silicon is only 10¹ A/cm². Accordingly, the very highresistance of a thin film amorphous silicon diode leads to long RC timeconstants and very low performance.

Cost estimates associated with MJT-MRAMs may be reduced dramaticallywith 3-D integration. For example, assuming three masks per layer andtwelve layers, lithography cost will increase by $1440 per wafer. If anadditional $1560 is allowed for other expenses, cost increases by 60percent, but density increases by a factor of 12. Nevertheless, despitegood 3-D potential, MRAM has poor scaling and does not appearcompetitive with other storage methods.

MATRIX

A MATRIX memory cell has one anti-fuse and one poly-crystalline silicondiode. See, for example, T. H. Lee, “A Vertical Leap for Microchips,”Scientific American, vol. 286, p. 52, 2002. MATRIX memory should have3-D integration costs that are similar to 3-D MRAM with the advantage ofbeing much more scalable. MATRIX memory, currently in development byMatrix Semiconductor, is the most advanced concept for 3-D solid-statememory having chips nearing production and being considered forcommercial use. The primary disadvantages of Matrix memory are: (1) thememory is write-once because it is based on destructive breakdown of aninsulator, and (2) the memory has low performance because apoly-crystalline silicon diode is used.

1T-FeRAM

A 1T-FeRAM memory cell consists of one MOSFET and one ferroelectriccapacitor having a hysteresis loop similar to the exemplary hysterisisloop 200 shown in FIG. 2. 1T-FeRAM memory is very similar to DRAM exceptthat the capacitor dielectric is replaced by ferroelectric material anda slightly different architecture is used. See, for example, O. Aucielloet al., “The Physics of Ferroelectric Memories,” Physics Today, vol. 51,p. 22, 1998. Use of the ferroelectric material in place of a dielectricmaterial has several advantages, such as (1) the capacitor isnon-volatile and does not need to be refreshed, (2) the capacitor canstore about 100 times more charge in the same amount of space, and (3)the capacitor is radiation hardened because the polarization of theferroelectric is not easily affected by radiation.

The quality of having radiation-hardness allows the charge limitassociated with a 1T-FeRAM memory cell to be reduced below a millionelectrons when the sensing method is changed so that current is detectedor a gain cell is used. See, for example, D. Takashima, “Overview andTrend of Chain FeRAM Architecture,” IEICE. Trans. Electron., vol. E84-C,p. 747, 2001. Consequently, 1T-FeRAM does not suffer from the scalingproblems associated with DRAM memory. Even though a ferroelectricmaterial is polycrystalline, it should be capable of scaling to 10 nm.In that regard, it has been calculated that ferroelectric grains ofPb(Zr, Ti)O₃ (PZT) as small as 2.5 nm are thermally stable. See, forexample, T. Yamamoto, “Calculated Size Dependence of FerroelectricProperties in PbZrO₃—PbTiO₃ System,” Integrated Ferroelectrics, vol. 12,p. 161, 1996. Additionally, ferroelectric PZT films as thin as 4 nm havebeen grown. See, for example, T. Tybell et al., “Ferroelectricity inThin Perovskite Films,” Applied Physics Letters, vol. 75, p. 856, 1999.Moreover, low-leakage polycrystalline ferroelectric capacitors as thinas 13 nm have been formed. See, for example, T. Kijima et al.,“Si-Substituted Ultrathin Ferroelectric Films,” Jpn. J. Appl. Phys.,vol. 41, p. L716, 2002. Lastly, lateral ferroelectric domains as smallas 6 nm have been switched with scanned probes. See, for example, Y. Choet al., “Tbit/inch² Ferroelectric Data Storage Based on ScanningNonlinear Dielectric Microscopy,” Applied Physics Letters, vol. 81, p.4401, 2002.

If the number of grains or domain wall pinning sites is sufficientlylarge in a single capacitor, it should be possible to store two or morebits per cell, but this should be difficult because the intermediatestate of the cell cannot be verified without destroying the state. Forthat reason, FIG. 1 indicates that 1T and 0T-FeRAM can scale to 10 nm,but will be limited to only one bit per cell.

Thus, 1T-FeRAM appears to have a good chance of replacing DRAM becauseof it has similar performance and better scalability. The need forhigher dielectric constants has already caused the DRAM industry toextensively investigate perovskite materials.

What is needed is a high-performance non-volatile solid-state memorythat scales well and allows for low-cost 3-D integration.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a high-performance non-volatilesolid-state memory that scales well and allows for low-cost 3-Dintegration.

A present invention provides a memory device that includes a pluralityof bit lines, a plurality of memory layers, a plurality of treestructures, and a plurality of plate line groups corresponding with atleast one memory layer. The plurality of bit lines is formed on asubstrate and is arranged substantially in a first plane and extendingsubstantially in a first direction. Each layer of the plurality oflayers has an array of memory cells, such as ferroelectric-capacitormemory cells or hysteretic-resistor memory cells. The plurality of treestructures is arranged in a plurality of rows of tree structures and aplurality of tree structures correspond to each bit line. Each treestructure has a trunk portion and at least one branch portion. Eachbranch portion of a tree structure corresponds to at least one layer.The trunk portion of each tree structure extends from the substrate,each branch portion of a tree structure extends from the trunk portionof the tree structure, and each tree structure corresponds to aplurality of layers. The trunk portion of at least one tree structureincludes a plurality of vias that are in line with each other.Alternatively, the trunk portion of at least one tree structure includesa plurality of vias, and at least one via is offset from at least oneother via. Each respective plate line group includes a plurality ofplate lines that extend in a direction that is substantiallyperpendicular to the first direction and overlap the branch portion ofeach tree structure in at least one row of tree structures at aplurality of intersection regions. A memory cell is located at eachintersection region in a layer. At least one memory cell can containferroelectric material with a pillar-like structure. Alternatively, aplurality of memory cells can be connected by a ferroelectric wire.Alternatively, a plurality of memory cells can be connected by aferroelectric film. As yet another alternative, at least one memory cellcan be a hysteretic-resistor memory cell. Additionally, each respectiveplate line group is arranged in a direction that is substantiallyperpendicular to a direction the branch portion of each tree structureextends.

A plurality of cell layer lines extends substantially in the firstdirection. A plurality of cell column lines extend in a direction thatis substantial perpendicular to the first direction and overlap theplurality of cell layer lines at a plurality of second intersectionregions. A plurality of plate-line driver transistors is arranged in atwo-dimensional array. Each respective plate-line driver transistorcorresponds to and is located at a respective second intersectionregion. A connection is formed between each plate line and theplate-line driver corresponding to the plate line. Each plate-linedriver transistor has a control terminal and the cell column line thatcorresponds to each plate-line driver transistor is coupled to thecontrol terminal of the plate-line driver transistor. Alternatively, thecell layer line that corresponds to each plate line drive transistor iscoupled to the control terminal of the plate-line driver transistor.

The memory device can also includes at least one tree having a pluralityof associated memory cells containing a first value and at least onetree having a plurality of associated memory cells containing a secondvalue that is different from the first value. An averaging circuitoutputs an average of an output of each tree having a plurality ofassociated memory cells containing the first value with an output ofeach tree having a plurality of associated memory cells containing thesecond value, and a comparator circuit determines an output value of aselected memory cell of the memory device based on the output of theaveraging circuit.

In an alternative configuration of the present invention, the memorydevice includes a plurality of access lines and a plurality of accesstransistors. The plurality of access lines is formed on the substrate.The access lines extend in a direction that is substantiallyperpendicular to the first direction and overlap the bit lines at aplurality of second intersection regions. Each second intersectionregion corresponds to a tree structure, and each access line correspondsto a tree structure row. Each respective access transistor correspondsto and is located at a respective second intersection region. Eachrespective access transistor is electrically disposed between the treestructure and the bit line corresponding to the second intersectionregion. Each respective access transistor has a control terminal and isfurther coupled to the access line corresponding to the secondintersection region.

In yet another alternative configuration of the present invention, eachaccess line is a write line and each access transistor is a writetransistor. Accordingly, the memory device further includes a pluralityof read transistors, a plurality of read lines, and a plurality of gaintransistors. The plurality of read transistors are each electricallydisposed between a tree structure and the bit line corresponding to thetree structure. The plurality of read lines is formed on the substrateand extends in a direction that is substantially perpendicular to thefirst direction, thereby overlapping the bit lines at a plurality ofthird intersection regions. Each third intersection region correspondsto a tree structure, and each read line corresponds to a tree structurerow. Each respective read transistor corresponds to and is located at arespective third intersection region. Each respective read transistor isfurther electrically disposed between the tree structure and the bitline corresponding to the third intersection region. Each respectiveread transistor has a control terminal and is coupled to the read linecorresponding to the third intersection region. Each gain transistorcorresponds to a read transistor and is disposed between the readtransistor and the tree structure corresponding to the read transistor.Each gain transistor includes a control terminal that is coupled to thecorresponding tree structure.

Another embodiment of the present invention provides a memory devicehaving a plurality of bit lines, a three-dimensional memory having aplurality of layers, a plurality of plate line groups, and a pluralityof plate-line driver transistors. Each layer of the three-dimensionalmemory includes an array of memory cells. Each memory cell includes aferroelectric capacitor. Alternatively, each memory cell can include ahysteretic-resistor memory cell. The plurality of bit lines is formed ona substrate, is arranged substantially in a first plane and extendssubstantially in a first direction. The plate line groups each include aplurality of plate lines and correspond to at least one layer of thethree-dimensional memory. The plurality of plate-line driver transistorsis formed on the substrate and is arranged in a two-dimensional array.Each plate-line driver transistor corresponds to a plate line. Aplurality of cell layer lines extends substantially in the firstdirection. A plurality of cell column lines extends in a direction thatis substantial perpendicular to the first direction and overlaps theplurality of cell layer lines at a plurality of second intersectionregions. Each respective plate-line driver transistor corresponds to andis located at a respective second intersection region. A connection isformed between each plate line and the plate-line driver correspondingto the plate line.

Yet another embodiment of the present provides a method for reading anderasing a memory device that includes a plurality of bit lines, aplurality of layers, a plurality of tree structures, and a plurality ofplate line groups corresponding to at least one layer. The plurality ofbit lines is formed on a substrate and is arranged substantially in afirst plane and extending substantially in a first direction. Each layerof the plurality of layers has an array of ferroelectric capacitormemory cells. The plurality of tree structures is arranged in aplurality of rows of tree structures and a plurality of tree structurescorrespond to each bit line. Each tree structure has a trunk portion andat least one branch portion. Each branch portion of a tree structurecorresponds to at least one layer. The trunk portion of each treestructure extends from the substrate, each branch portion of a treestructure extends from the trunk portion of the tree structure, and eachtree structure corresponds to a plurality of layers. The trunk portionof at least one tree structure includes a plurality of vias that are inline with each other. Alternatively, the trunk portion of at least onetree structure includes a plurality of vias, and at least one via isoffset from at least one other via. Each respective plate line groupincludes a plurality of plate lines, extends in a direction that issubstantially perpendicular to the first direction and overlaps thebranch portion of each tree structure in at least one row of treestructures at a plurality of intersection regions. A ferroelectriccapacitor memory cell is located at each intersection region in a layer.Additionally, each respective plate line group is arranged in adirection that is substantially perpendicular to a direction the branchportion of each tree structure extends. According to the invention, eachtree structure in the at least one row is allowed to electrically floatnear a first predetermined voltage. A second predetermined voltage V isapplied to a selected plate line. A potential or change in potential ofeach tree structure in the at least one row is detected and it isdetermined whether each detected potential or potential changecorresponds to a 0 or a 1 for each memory cell at the intersections ofthe selected plate line and the tree structures in the at least one row.The first predetermined voltage is applied to every tree structure inthe at least one row; and the first predetermined voltage is applied tothe selected plate line.

Still another embodiment of the present provides a method for reading,erasing, and rewriting a memory device that includes a plurality of bitlines, a plurality of layers, a plurality of tree structures, and aplurality of plate line groups corresponding to at least one layer. Theplurality of bit lines is formed on a substrate and is arrangedsubstantially in a first plane and extending substantially in a firstdirection. Each layer of the plurality of layers has an array offerroelectric capacitor memory cells. The plurality of tree structuresis arranged in a plurality of rows of tree structures and a plurality oftree structures correspond to each bit line. Each tree structure has atrunk portion and at least one branch portion. Each branch portion of atree structure corresponds to at least one layer. The trunk portion ofeach tree structure extends from the substrate, each branch portion of atree structure extends from the trunk portion of the tree structure, andeach tree structure corresponds to a plurality of layers. The trunkportion of at least one tree structure includes a plurality of vias thatare in line with each other. Alternatively, the trunk portion of atleast one tree structure includes a plurality of vias, and at least onevia is offset from at least one other via. Each respective plate linegroup includes a plurality of plate lines, extends in a direction thatis substantially perpendicular to the first direction and overlaps thebranch portion of each tree structure in at least one row of treestructures at a plurality of intersection regions. A ferroelectriccapacitor memory cell is located at each intersection region in a layer.Additionally, each respective plurality of plate lines is arranged in adirection that is substantially perpendicular to a direction the branchportion of each tree structure extends. According to the invention, eachtree structure in the at least one row is allowed to electrically floatnear a first predetermined voltage. A second predetermined voltage V isapplied to a selected plate line. A potential or change in potential ofeach tree structure in the at least one row is detected and it isdetermined whether each detected potential or potential changecorresponds to a 0 or a 1 for each memory cell at the intersections ofthe selected plate line and the tree structures in the at least one row.The first predetermined voltage is applied to every tree structure inthe at least one row; and the first predetermined voltage is applied tothe selected plate line. According to the invention, a voltage V/3 isapplied to each tree structure in the at least one row of treestructures. A voltage 2V/3 is applied to each plate line in the at leastone row of tree structures. Then, a voltage V is applied to apredetermined number of selected tree structures in the at least one rowof tree structures. A voltage 0 is applied to a selected plate linewhere a data “1” will be written in a predetermined number of selectedmemory cells at the intersection of the first predetermined number oftree structures and the selected plate line. A voltage 2V/3 is appliedto the selected plate line and a voltage V/3 is applied to thepredetermined number of selected tree structures in the at least one rowof tree structures. A voltage 0 is applied to each plate line in the atleast one row of tree structures, and a voltage 0 is applied to eachtree structure in the at least one row of tree structures.

Another embodiment of the present invention provides a method forreading and erasing a memory device. The memory device includes aplurality of bit lines, a three dimensional memory, a plurality of plateline groups and a plurality of plate-line driver transistors. Theplurality of bit lines is formed on a substrate, is arrangedsubstantially in a first plane and extends substantially in a firstdirection. The three-dimensional memory includes a plurality of layersand a plurality of tree structures. Each layer has a plurality of memorycells and the tree structures are arranged in a plurality of rows. Eachtree structure has a trunk portion and at least one branch portion. Eachbranch portion of a tree structure corresponds to at least one layer,and each branch portion of a tree structure extends from the trunkportion of the tree structure. The plate line groups each include aplurality of plate lines that correspond to at least one layer of thethree-dimensional memory. Each respective plate line group overlaps thebranch portion of each tree structure in at least one row of treestructures at a plurality of intersection regions. A memory cell, suchas a ferroelectric memory cell, is located at each intersection regionin a layer. The plurality of plate-line driver transistors is formed onthe substrate and is arranged in a two-dimensional array. Eachplate-line driver transistor corresponds to a plate line. A plurality ofcell layer lines extends substantially in the first direction. Aplurality of cell column lines extends in a direction that issubstantial perpendicular to the first direction and overlaps theplurality of cell layer lines at a plurality of second intersectionregions. Each respective plate-line driver transistor corresponds to andis located at a respective second intersection region. A connection isformed between each plate line and the plate-line driver correspondingto the plate line. According to the invention, each tree structure in atleast one row is allowed to electrically float near a firstpredetermined voltage. A second predetermined voltage V is applied to aselected plate line. A potential or change in potential of each treestructure in the at least one row is detected and it is determinedwhether each detected potential or potential change corresponds to a 0or a 1 for each memory cell at the intersections of the selected plateline and the tree structures in the at least one row. The firstpredetermined voltage is then applied to every tree structure in the atleast one row and to the selected plate line.

Another embodiment of the present invention provides a method forwriting data to a previously erased memory device. The memory deviceincludes a plurality of bit lines, a three dimensional memory, aplurality of plate line groups and a plurality of plate-line drivertransistors. The plurality of bit lines is formed on a substrate, isarranged substantially in a first plane and extends substantially in afirst direction. The three-dimensional memory includes a plurality oflayers and a plurality of tree structures. Each layer has a plurality ofmemory cells and the tree structures are arranged in a plurality ofrows. Each tree structure has a trunk portion and at least one branchportion. Each branch portion of a tree structure corresponds to at leastone layer, and each branch portion of a tree structure extends from thetrunk portion of the tree structure. The plate line groups each includea plurality of plate lines and correspond to at least one layer of thethree-dimensional memory. Each respective plate line group overlaps thebranch portion of each tree structure in at least one row of treestructures at a plurality of intersection regions. A memory cell, suchas a ferroelectric memory cell, is located at each intersection regionin a layer. The plurality of plate-line driver transistors is formed onthe substrate and is arranged in a two-dimensional array. Eachplate-line driver transistor corresponds to a plate line. A plurality ofcell layer lines extends substantially in the first direction. Aplurality of cell column lines extends in a direction that issubstantial perpendicular to the first direction and overlaps theplurality of cell layer lines at a plurality of second intersectionregions. Each respective plate-line driver transistor corresponds to andis located at a respective second intersection region. A connection isformed between each plate line and the plate-line driver correspondingto the plate line. According to the present invention, a voltage V/3 isapplied to each tree structure in at least one row of tree structures. Avoltage 2V/3 is applied to each plate line in the at least one row oftree structures. A voltage V is applied to a predetermined number ofselected tree structures in the at least one row of tree structures. Avoltage 0 is applied to a selected plate line where a data “1” will bewritten in a predetermined number of selected memory cells at theintersection of the first predetermined number of tree structures andthe selected plate line. A voltage 2V/3 is applied to the selected plateline. A voltage V/3 is applied to the predetermined number of selectedtree structures in the at least one row of tree structures. A voltage 0is applied to each plate line in the at least one row of treestructures, and to each tree structure in the at least one row of treestructures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not bylimitation in the accompanying figures in which like reference numeralsindicate similar elements and in which:

FIG. 1 shows a table setting forth estimated performance characteristicsand estimated costs at the limits of scaling for current and potentialsolid-state memory technologies, as projected for the year 2020;

FIG. 2 shows an exemplary hysterisis loop for illustratingcharacteristics of FeRAM-based memory cells;

FIG. 3 shows a cross-sectional view of a first exemplary arrangement ofa first embodiment of a 3-D 0T-FeRAM memory according to the presentinvention, as viewed along line B-B shown in FIG. 4;

FIG. 4 shows a cross-sectional view of the exemplary 3-D 0T-FeRAM memoryshown in FIG. 3, as viewed along line A-A in FIG. 3;

FIG. 5 depicts an overall chip layout of the exemplary memory shown inFIGS. 3 and 4;

FIG. 6 shows a timing diagram illustrating a first read operation for areading memory cell of a 3-D 0T-FeRAM memory according to the presentinvention;

FIG. 7 shows a timing diagram illustrating a second, alternative readoperation for reading a memory cell of a 3-D 0T-FeRAM memory accordingto the present invention;

FIG. 8 shows a cross-sectional view of an alternative exemplaryarrangement of the first embodiment of a 3-D 0T-FeRAM memory accordingto the present invention;

FIG. 9 shows a cross-sectional view of an exemplary arrangement of asecond embodiment of a 3-D 0T-FeRAM memory according to the presentinvention;

FIG. 10 shows a cross-sectional view of an exemplary arrangement of athird embodiment of a 3-D 0T-FeRAM memory according to the presentinvention;

FIG. 11 shows a cross-sectional view of an exemplary arrangement of afourth embodiment of a 3-D 0T-FeRAM memory according to the presentinvention;

FIG. 12 shows a cross-sectional view of an exemplary alternativearrangement of active components in the substrate below a 3-D memoryaccording to the present invention;

FIG. 13 depicts ferroelectric memory cells formed by a first techniqueaccording to the present invention;

FIG. 14 depicts ferroelectric memory cells formed by a second techniqueaccording to the present invention;

FIG. 15 depicts ferroelectric memory cells formed by a third techniqueaccording to the present invention;

FIG. 16 is a schematic diagram illustrating an exemplary circuit fordetecting the output of a selected memory cell according to the presentinvention; and

FIGS. 17-19 depict an arrangement of a 3-D memory according to thepresent invention having tree structures that are placed only 2 F apart,in which F is the lithography half-pitch.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an ultra-low-cost, scalable,nonvolatile solid-state memory having a very high density, a highperformance, and very low power dissipation. In particular, the presentinvention relates to a three-dimensional (3-D) arrangement of 0T-FeRAMmemory cells, in which each memory cell includes a ferroelectriccapacitor. The memory cells are arranged in a tree-like structure withcross-point access being through the base of the tree and throughplate-lines threading at least one row of trees. The trees have built-insense gain and cell disturbance is managed by sequential access within atree row. Multiple layers of ferroelectric capacitors are built on topof a single active silicon layer containing access transistors, gaintransistors, sense circuitry, and a two-dimensional array of plate-linedrivers. The architecture is designed to have the lowest possiblefabrication cost with memory layers comprising only ferroelectricmaterial between crossed wires. Due to the special arrangement for oneembodiment of the present invention, the same three masks may berepeatedly used for defining all memory layers and the total number ofmask steps is about 3N+1, in which N is the number of layers. Otherembodiments of the present invention require fewer mask steps.

3-D integration of 0T-FeRAM is readily achievable because the memorylayer of a memory cell is formed by a ferroelectric material betweencrossed wires. See, for example, T. Nishihara et al., “A Quasi-MatrixFerroelectric Memory for Future Silicon Storage,” IEEE Journal ofSolid-State Circuits, vol. 37, p. 1479, 2002. Such a configuration is nomore complicated than conventional back-end processing performed formicroprocessors in which there are typically eight levels of wiringbuilt on top of the transistors of the microprocessor. Moreover,ferroelectric materials can be grown at temperatures below 600 C makingferroelectric materials suitable for back-end processing.

A 3-D memory based on 0T-FeRAM has been previously disclosed by T.Nishihara et al., supra. As disclosed, each layer of memory wasconnected to the bottom silicon layer through a separate via, resultingin a large amount of space being consumed by the vias. Thus, datadensity was decreased while cost was increased. Separate vias alsoincrease mask complexity requiring different masks for defining commonelectrodes and vias in every layer, therefore greatly increasing maskcosts. Layer selector transistors were also used for individuallyaccessing each layer. The transistors occupied valuable silicon realestate and could increase the minimum sector size that can beindividually accessed without disturbance problems. When the number ofcells connected by disturbance is larger, the energy required to writeis larger because more plate lines must be energized during the writeprocess.

Plate lines in the Nishihara et al memory device were connected betweenlayers in the same column, thereby complicating a reading process.Moreover, because no timing diagram was disclosed, it was not clear asto how data was intended to be read. One potential approach could be toswitch a plate line high and sequentially read each bit connected to aplate line by turning on each unit transistor. Besides beingcomplicated, there is the danger of cross talk and disturbance occurringduring the readout because voltages on common electrodes in a layercould couple through the capacitors to the plate lines connecting thedifferent layers.

According to one embodiment of the present invention, each layer of0T-FeRAM memory can be fabricated using only three masks. If EUVlithography is used having an estimated cost of $40 per mask step perwafer, 16 layers would add an estimated $1920 to the $5000 price perwafer. This is likely to be the dominant cost adder. If an additional$1080 is added for other processing and testing costs, then theestimated cost of the chip increases by 60 percent, but the memorydensity increases by a factor of 16. This results in a cost per bit thatis ten times lower than a 2-D memory structure. According to anotherembodiment of the present invention, 16 layers of memory could befabricated with as few as 27 mask steps and the cost per bit could besignificantly lower. Also, because of the simple, self-aligning 3-Dstructure of the embodiments of the present invention, it is possiblethat less expensive lithography may be used such as imprint lithography.

A 0T-FeRAM memory cell is the simplest of all solid-state storagemethods in FIG. 1, having only a single capacitor per cell. Current doesnot pass through the cell for either writing or reading so no othercircuit elements are needed in the cell for steering current. The ideaof a simple cross-bar of ferroelectric capacitors was popular in theearly days of FeRAM, but was abandoned because of disturbance issues.See, for example, O. Auciello et al., supra. Disturbances occur whenvoltages are applied to bit lines and word lines for accessing a crosspoint. Smaller voltages may be inadvertently applied across unselectedcapacitors, thereby causing an unselected cell to pass through a minorpolarization loop, as shown by 201 in FIG. 2, and lose polarization.This problem can be reduced by using improved ferroelectric materialsthat have better hysteresis loop squareness. MRAM writing isaccomplished by a half-select scheme in which half the select field isprovided by the bit line and half from the word line. Half-selectedcells do not usually switch in MRAM because magnetic materials have verysquare hysteresis loops.

Another way to reduce disturbances is to limit the number of cells thatare disturbed and rewrite a cell each time the cell is disturbed. Inpractice, the data is read and rewritten sequentially until all the datathat have been linked by disturbance are read. Similar to DRAM, a FeRAMread operation is destructive and the read data must, in any event, berewritten. See, for example, T. Nishihara et al., “A Quasi-MatrixFerroelectric Memory for Future Silicon Storage,” IEEE Journal ofSolid-State Circuits, vol. 37, p. 1479, 2002, and U.S. Pat. No.6,301,145 to T. Nishihara, entitled “Ferroelectric Memory and Method forAccessing Same.”

Data is read in “sectors” and byte access is not available, similar toHDD and FLASH memories. The maximum number of disturbance pulses islimited to the number of cells connected together when the starting cellis always the same, or is limited to about twice the number of connectedcells when the starting cell is random (to gain faster access to aparticular byte of data).

FIG. 3 shows a cross-sectional view of a first exemplary arrangement ofa first embodiment of a 3-D 0T-FeRAM memory 300 according to the presentinvention, as viewed along line B-B, which is shown in FIG. 4.Specifically, FIG. 3 is a cross-sectional view showing details of theend of two rows of trees. FIG. 4 shows a cross-sectional view of thefirst exemplary arrangement of 3-D 0T-FeRAM memory 300 as viewed alongline A-A in FIG. 3. Specifically, FIG. 4 is a cross-sectional view ofmemory 300 showing details of plate lines through a row of trees.

Memory 300 includes a plurality of memory cells 301 that are each formedfrom a single ferroelectric capacitor and that are arranged in atree-like structure, referred to herein as a “memory tree.” Not allmemory cells 301 are indicated for simplification of FIGS. 3 and 4.

Two memory trees 302 a and 302 b are shown in FIG. 3. Memory trees 302 aand 302 b are arranged to be mirror images of each other so that acommon voltage line 303 can be shared for gain transistors 304 a and 304b. Gain transistors 304 a and 304 b and read transistors 311 a and 311 bare used to convert a voltage on a tree structure into a current on abit line for improved detection sensitivity. As best viewed in FIG. 3,each memory tree 302 includes a base or “trunk” portion 305 that isformed from a conductive material, such as platinum, and a plurality of“branch” portions 306 that are also formed from a conductive material,such as platinum. In FIG. 3, each branch portion 306 forms a layer ofmemory 300. Memory cells 301 are arranged along branches 306. As shownin FIG. 3, the trunk portion 305 is a series of vias between layers usedto connect to branch portions 306 in a plurality of layers. For each ofthe embodiments of the present invention, the vias forming the trunkportion can be displaced relative to one another in different layerswithout changing the operation of the invention as long as the memorycells in a plurality of layers are connected by a conducting path.Therefore, the tree can have a variety of possible shapes. As bestviewed in FIG. 4, plate lines 307 are formed to thread between branches306. Each plate line 307 connects to a row of memory cells 301, therebyforming a 3-D cross-point array. Cross-point access to a particularmemory cell 301 is through a trunk portion 305 and branch portion 306and a plate-line 307 corresponding to the memory cell. A via 308connects each plate line of a row of trees to a 2-D array of plate-linedriver transistors 309.

Although only four layers are shown in FIGS. 3 and 4, at least 16 layerscan be added to a tree structure by simply adding each additional layerto the top of the tree. Each additional layer increases the effectivememory density.

Multiple layers of storage cells are built on top of a single activesilicon layer 310, which contains gain transistors 304 a and 304 b, readtransistors 311 a and 311 b, write transistors 312 a and 312 b, and the2-D array of plate-line drivers 309. Each tree 302 a and 302 b hasbuilt-in sense gain by respectively connecting trunk portions 305 a and305 b to gates 318 a and 318 b of gain MOSFETs 304 a and 304 b throughconductive branches 314 a and 314 b. The potential of the memory tree ismeasured by turning on a read transistor 311 during a read operation andmeasuring the current flowing on bit line 313 a, which connects tomultiple rows of trees. A write transistor 312 is used to place voltageson a memory tree during a write operation. Cell disturbance is managedby sequential access within a tree row. That is, data is read andwritten by accessing each plate line in a serial manner until all memorycells in a row of trees are accessed.

The same three masks may be used repeatedly for defining all memorylayers of the first embodiment of the present invention. This isdepicted in FIG. 4, which shows a side view of the end of a row of treeswhere plate lines 307 are connected to the 2-D array of plate-linedrivers 309. One mask is used to make memory tree branches 306. One maskis used to make plate lines 307, and one mask make is used to make vias308. The plate line mask and the via mask can be offset in each layer tomake connections to plate-line drivers 309. Thus, plate lines 307 aremade slightly offset at each higher layer of the memory tree. Offsettingof the via mask for each higher layer creates extra partial vias 315that do not adversely affect operation of memory 300. Offsetting masksalso causes partial trees at the opposite end of the row of trees, butthese partial trees do not adversely affect operation of memory 300 withproper design of the lower non-repeated layers. Alternatively, differentmasks may be used for each layer so that extra vias 315 and partialtrees do not occur.

Plate-line driver transistors 309 form a 2-D array and are addressed bya plurality of cell layer lines 316 and a plurality of cell column lines317. FIG. 4 also shows bit lines 313 a-313 d, while FIG. 3 only showsbit line 313 a for clarity. Bit lines 313 a-313 d are arranged to snakethrough trunks 305 at two different levels so that trees 302 can be asclose together as possible.

FIG. 8 shows a cross-sectional view of a second exemplary arrangement ofthe first embodiment of a 3-D 0T-FeRAM memory according to the presentinvention. Memory 800 includes a plurality of ferroelectric capacitormemory cells 801 that are arranged in a memory tree. Not all memorycells 801 are indicated for simplification of FIG. 8. Two memory trees802 a and 802 b are shown in FIG. 8. Memory trees 802 a and 802 b areshown as mirror images of each other, but do not necessary need to be.Each memory tree 802 includes a trunk portion 805 that is formed from aconductive material, such as platinum, and a plurality of branchportions 806 that are also formed from a conductive material, such asplatinum. Each branch portion 806 forms a layer of memory 800. Memorycells 801 are arranged along branches 806. As shown in FIG. 8, trunkportions 805 are a series of vias between layers used to connect tobranch portions 806 in a plurality of layers. Similar to memory 300shown in FIGS. 3 and 4, the vias forming trunk portions 805 can bedisplaced relative to one another in different layers without changingthe operation of the invention as long as memory cells 801 in aplurality of layers are connected by a conducting path. Consequently,each tree structure 805 can have a variety of possible shapes. Platelines 807 are formed to thread between branches 806 of other treestructures that are not shown in FIG. 8. Although only four layers areshown in FIG. 8, at least 16 layers can be added to a tree structure bysimply adding each additional layer to the top of the tree.

The arrangement of memory 800 differs from the arrangement of memory300, shown in FIGS. 3 and 4, by not having a gain transistor forimproving memory cell detection sensitivity, and by not having separateread and write lines. Instead, memory 800 includes an access line 820that is used for accessing a particular memory cell 801 through anaccess transistor 811 and a bit line 813 for both reading and writingoperations.

FIG. 5 depicts an overall chip layout 500 of exemplary memory 300. Alarge number of trees, for example as many as 1024, are lined up to forma tree row 501. A plurality of tree rows 501 are arranged in a treearray portion 502 of chip layout 500. An array 503 of plate-line drivertransistors 309 is located at one end of the tree rows. The plate-linedriver transistors 309 are selected using cell layer lines 316, whichare driven by cell layer line drivers located in cell layer driver arrayportion 504, and by cell column lines 317, which are driven by cellcolumn drivers located in cell column and read/write driver arrayportion 505. Each row of trees has its own write line 320 and read line321, which are respectively driven by write and read drivers that arealso located in cell column and read/write driver array portion 505.Note that the connections from the write line 320 and read line 321 tothe read/write driver array are not shown in FIG. 4 for the sake ofsimplicity. Also note that the write and read drivers may also belocated on the left edge of the tree array portion 502 of chip layout500. Bit lines 313 thread through a large number of tree rows, forexample, as many as 128 rows, and are driven by bit line drivers andsense amplifiers driver array 506, which is located at the end of bitlines 313.

FIG. 6 shows a timing diagram illustrating a first read operation 600for reading memory cell 301 b of 3-D 0T-FeRAM memory 300 shown in FIGS.3-5. A read operation 600 is divided into a read phase 601 and awrite-back phase 602. A V/3 disturbance sequence and a sequential accesssequence in a sector are used for preventing disturbance-relatedproblems.

During read phase 601, a read line (RL), such as read line 321 b (FIG.3) shown in FIG. 3, is turned on at 603 causing an offset current toflow on bit line (BL), such as bit line 313 a, at 604. At 606, a cellcolumn select line (CC1), such as cell column select line 317 b, isturned on. At 605, a voltage V is applied to one of the cell layer linesthrough the plate-line driver and onto one of the plate lines (PL11),such as plate line 307 b, to write 0 on all storage cells in the treerow connected to that plate line. (The notation PL11 indicates the plateline physically located at layer 1 in column 1, i.e., towards the lowerright corner of FIG. 3.) The other cell column transistors (CC2)(indicated at 607) must be off to prevent the other plate lines in theselected layer, such as PL12, from going high to voltage V. Plate linesPLX2 in column 2, in which “X” indicates any layer, are also floating,as shown at 608.

When selected memory cell 301 b contains a “0”, a small amount of chargeis placed on the memory tree (MT) 302 b at 609 a, causing the voltage ofmemory tree 302 b to rise a small amount. In contrast, when the memorycell 302 b contains a “1”, the polarization switches and a larger amountof charge is placed on memory tree 302 b at 609 b, thereby causing alarger change in potential and a larger change in the conductance ofgain transistor 304 b. Sense amplifiers (not shown) that are connectedto bit line 313 a sense whether the bit line current or change in bitline current is large enough to be a “1” based on known “0”s and “1”slocated in extra sample trees (not shown). The result is stored by thesense amplifiers for write back phase 602. All bits along the selectedplate line (for example, a total of 1024 bits) are detectedsimultaneously. Next during read phase 601, a “0” is reinforced bydischarging memory tree 302 b at 610.

Write-back phase 602 begins by bringing all of the memory trees 302 in atree row to V/3, as shown at 611. At this point, all of the plate lines306 in non-accessed columns are brought to 2V/3 (PLX2) at 612. The othercell column transistors CC2 are then turned off at 613 and PL21 isbrought to 2V/3 at 614, while PL11 is kept at 0 (615). When memory cell301 b contained a “0”, memory tree 302 b is kept at V/3 at 616 a. Whenmemory cell 301 b contained a “1”, a voltage V is applied to memory tree302 b (at 616 b). All memory cells on a plate line (for example, 1024bits) are written simultaneously. Disturbance voltages are kept to amaximum of V/3. Nevertheless, because PLX2 is floating while the memorytrees are pulsed, capacitive coupling could possibly increase thepotentials on the plate lines. Consequently, the disturbance voltagedifference could be greater than V/3 in memory trees that are kept atV/3.

FIG. 7 shows a timing diagram illustrating a second, alternativepreferred read operation 700 for reading memory cell 301 b of 3-D0T-FeRAM memory 300 shown in FIGS. 3-5. Read operation 700 also includesa read phase 701 and a write-back phase 702. Read phase 701 is identicalto read phase 601, shown in FIG. 6, and is labeled similarly. Thebeginning of write-back phase 702 is similar to write-back phase 602 bybringing all of the memory trees 302 in a tree row to V/3, as shown at711. Afterward, all plate lines 307 are brought to 2V/3, shown at 712 a,712 b and 712 c. At 713 b, the memory trees 302 in the tree row are thenbrought to either V when memory cell 301 b contained a “1”, or left atV/3 when memory cell 301 b contained a “0” at 713 a. At 714, the plateline 307 b is pulsed to 0. Even though plate lines PLX2 s are floatingduring the pulse, they are not affected because the memory trees are notfloating or changing potential. Again, all memory cells on plate line307 b (for example, a total of 1024 bits) are written simultaneously.

The same process is repeated for the next plate line, regardless whetherthe read operation of FIG. 6 or FIG. 7 is used, until all plate lines ina tree row are read and written. When data is to be written, only thewrite-back phase of either FIG. 6 or FIG. 7 is used, thereby omittingthe read phase of the operation. Operation of memory 800 (FIG. 8) isshown by the timing diagrams shown in FIGS. 6 and 7, with the BL(V)output being equal to signal MT in both Figures. Signal BL(I) would notappear for memory 800. Also, the access transistor is turned on for boththe read and write operations and the memory tree voltage (MT) is passedto the bit line voltage (BL) for both the read and write operations.

In reference to memory 800, it is possible that the memory cells 801 arenot simple ferroelectric capacitors. Rather than having a hystereticpolarization loop with electric field (as in FIG. 2), the memory cellmay have a hysteretic resistance with electric field. That is,application of a voltage across the device in one direction leads to ahigh resistance and application of a voltage in the other directionleads to a low resistance. One example of this type of device isdescribed in W. W. Zhuang et al., “Novell Colossal Magnetoresistive ThinFilm Nonvolatile Resistance Random Access Memory (RRAM),” IEDM Tech.Digest (IEEE), p. 193 (2002). With hysteretic resistance memory andusing the architecture shown in FIG. 8, data may be read by applying asmaller, nondestructive voltage to the plate line at 605 or 705 (replaceV with a smaller voltage V* in FIG. 6 or FIG. 7). Because resistancemeasurement of a cell is desired, the state of the cell is determinednow by the current that flows to the bit line rather than the voltageand there is no need for a gain transistor. Accordingly, the timingdiagram for reading would look like what is shown in FIG. 6 or FIG. 7,except that MT would look like BL(V) and BL(I) would look the same as isshown. Also, the access transistor would be turned on for both read andwrite operations. This would be the case if, for example, the detectioncircuit held the bit line and memory tree at virtual ground whilecurrent was detected. For resistance memory, there exists thepossibility of offset current flowing from the memory tree back to anyof the other plate lines in the tree when there is any potentialdifference between the tree and any of the plate lines. Therefore, it isimportant that the offset voltages from the detection circuit and theoffset voltages from the circuitry that puts “ground” on thenon-accessed plate lines in the memory tree are as small as possible.The architecture shown in FIG. 8 is ideal for this type of memorybecause no diode or transistor is needed for every cell and the numberof leakage paths is minimized (for example, seven leakage paths in FIG.8). Those skilled in the art will also recognize that the detectioncircuit could also measure the leakage current before voltage V* isapplied to the plate line, store the measured leakage current, andsubtract the leakage current from the signal although this wouldcomplicate the detection circuitry significantly. The write sequencecould be the same as for ferroelectric memory. That is, memory cellsalong a plate line are first erased and then a “1” is written in somecells using the sequence shown in FIG. 6 or FIG. 7.

Hysteretic-resistor memory may have some advantages over ferroelectricmemory. First, reading is nondestructive so that data is not erased anddoes not have to be rewritten after the data is read. Second,intermediate values of resistance could be written. For example, withthe appropriate write voltages, one of four different values ofresistance could be stored to represent “00”, “01”, “10”, and “11” andtwo bits of information could be stored in each memory cell. This wouldlower the cost per bit by about a factor of two. For this type ofmemory, it is desirable to have a fairly high resistance even in the lowresistance state and keep current density through each cell lower thanabout 10⁴ A/cm² so that current can be supplied to a large number ofmemory trees in a row simultaneously through the plate line withoutelectromigration problems in the plate line and also avoid significantvoltage drops along the plate line.

For a memory tree arrangement having four cells per branch, 16 layersand with 1024 trees in a row, the minimum data sector size is 8 KB. Themaximum number of disturbances is 64 for a sequential-access operation,or 127 when the starting plate line is randomly selected. Disturbanceconditions do not occur in other tree rows that are not being accessed.

The performance characteristics of memory 300 can be estimated forcopper interconnects (i.e., plate lines with a copper core), and amemory cell having a ferroelectric capacitor, such as disclosed by T.Kijima et al., supra. For this example, the thickness of theferroelectric capacitor is s=13 nm, polarization P=20 μC/cm², anddielectric constant ∈=200. The voltage difference on the memory treebetween “1” and “0” when one capacitor switches is given by V=2 Ps/N∈∈₀,in which N is the number of capacitors in the tree. When N=64, thevoltage difference V is 46 mV. Assuming a 60 mV/decade transistorsub-threshold slope, the conductance of the gain transistor changes by afactor of six. Thus, the signal output from the gain transistor is largeenough to be easily detectable.

The speed of memory 300 is determined by plate line capacitance ratherthan ferroelectric switching time. For example, the switching time ofperovskites materials is much less than 1 ns. The capacitance of theplate line will be dominated by the large dielectric constant andcapacitance of the ferroelectric material. If no capacitors areswitching, the capacitance is C=Mεε₀ A/s, in which M is the number ofcapacitors on the plate line and A is the area of each capacitor(ignoring edge effects). If the capacitors are taken to be 11 nm in sizeand M=1024, then C=17 fF. If the switching voltage is taken to be 0.5 V,then the effective switching capacitance of each capacitor will be 2PA/V=0.1 fF. Consequently, if half of the capacitors switch, theeffective plate line capacitance will be 50 fF. If the maximum currentdensity is taken to be 10⁷ A/cm², then the maximum current though a22×11 nm wire will be 24 μA. Thus, the slew-rate-limited plate line risetime is CV/I=1 ns when half of the capacitors switch and 2 ns in theworst case when all of the capacitors switch.

The resistivity of copper wires at the nanoscale increases due tointerface scattering, so the resistivity is taken as ρ=5 μΩ-cm. Theresistance of a 22×11 nm copper wire is 5 kΩ for a length of 22 μm. TheRC time constant is 0.2 ns, so read and write times will be dominated bythe plate-line slew rate when capacitors are switching. Thus, theminimum time to read a particular byte of data is a little longer than 2ns, which is an access time that is about 1 million times faster than aHDD. This estimate also assumes that the sense amplifiers are fast incomparison to the plate line rise time. Referring to the timing diagramof FIG. 7, there are two plate line swings that cause switching (with upto 100 fF of capacitance) and four that do not (with 17 fF ofcapacitance) during a full read/write cycle. In FIG. 1, the minimum readaccess time is taken to be 5 ns and the minimum write time to be 10 ns.Consequently, the read/write cycle time is 15 ns for one plate line and1 μs for 8 KB of data. This means that the data rate is 8 GB/s for onetree array on a chip that may also operate many arrays in parallel foreven higher data rates.

Power requirements can be calculated based on CV² with capacitance Cbeing dominated by plate line swings, particularly during the writecycle in which all plate lines must be brought to 2V/3 and back to 0 forreduced disturbance effects. If ferroelectric materials can be improved,all plate lines other than PL11 (FIGS. 6 and 7) could be kept at V/2throughout the read/write cycle resulting in even better powerefficiency because only the active plate line and the memory trees wouldneed to be charged. Speed would also be higher, but disturbance voltageswould be V/2 instead of V/3. Power requirements could also be reduced byusing a ferroelectric material having a lower dielectric constant.

Even with the timing shown in FIG. 7, power requirements are much lowerthan any other type of memory listed in FIG. 1. For example, assuming 64plate lines and adding up the CV² energy for each plate line swing andmemory tree swing, 0.5 pJ are required to read and write 1024 bits in 15ns, which corresponds to 35 μW for 8 GB/s. This compares favorably to aHDD, which can supply a data rate of 200 MB/s with about 10 W of power.Thus, 0T-FeRAM could be on the order of 10 million times more energyefficient than a HDD.

In contrast to the power requirements for the present invention, thecurrent density through an OUM cell must be about 10⁷ A/cm² to heat up ananoscale volume to 600 C, while for reading, the current density isperhaps half this amount. For an OUM cell having a cross section of 11nm×11 nm, the current requirements are 12 μA. The voltage across thediode and phase change cell is about 1.5 V, so the power dissipation is18 μW. Because it takes at least 50 ns to write, the amount of energyrequired for OUM for a one bit write is 1 pJ. For multi-bit OUM,multiple pulses may be required to program and the total time wouldaccordingly be longer. Thus, 0T-FeRAM writing is 2000 times more energyefficient than for OUM in this example.

FIG. 9 shows a cross-sectional view of an exemplary arrangement of asecond embodiment of a 3-D 0T-FeRAM memory 900 according to the presentinvention. In particular, FIG. 9 is a cross-sectional view showingdetails of the end of two rows of trees. Memory 900 includes a pluralityof memory cells 901 that are each formed from a single ferroelectriccapacitor and that are arranged in a memory-tree structure. Not allmemory cells 901 are indicated for simplification of FIG. 9. Two memorytrees 902 a and 902 b are arranged to be mirror images of each other sothat a common voltage line 903 can be shared for gain transistors 904 aand 904 b. Gain transistors 904 and read transistors 911 are used forconverting a voltage on a tree structure into a current on a bit linefor improved detection sensitivity. Each memory tree 902 includes atrunk portion 905 that is formed from a conductive material, such asplatinum, and a plurality of branch portions 906 that are also formedfrom a conductive material, such as platinum. Each branch portion 906 isformed between two layers of memory cells 901. That is, memory cells 901are arranged above and below each branch portion 906 so that one branchportion 906 is connected to two layers of memory cells 901. Similar totrunk portions 305 of memory 300, trunk portions 905 are a series ofvias between layers that are used to connect to branch portions 906 andcan have a variety of possible shapes. Cross-point access to aparticular memory cell 901 is through a trunk portion 905 and branchportion 906 and a plate-line 907 corresponding to the memory cell.

Multiple layers of storage cells are built on top of a single activesilicon layer 910, which contains gain transistors 904 a and 904 b, readtransistors 911 a and 911 b, write transistors 912 a and 912 b, and a2-D array of plate-line drivers (not shown). Each tree 902 a and 902 bhas built-in sense gain by respectively connecting trunk portions 905 aand 905 b to gates 918 a and 918 b of gain MOSFETs 904 a and 904 bthrough conductive branches 914 a and 914 b. The potential of the memorytree is measured by turning on a read transistor 911 during a readoperation and measuring the current flowing on bit line 913 a, whichconnects to multiple rows of trees. A write transistor 912 is used toplace voltages on a memory tree during a write operation. Celldisturbance is managed by sequential access within a tree row. That is,data is read and written by accessing each plate line in a serial manneruntil all memory cells in a row of trees are accessed.

The first exemplary arrangement the first embodiment of a memory 300,shown in FIG. 3, requires 3N+1 masks, in which N is the number of memorylayers. For each layer, one mask is used for the tree branch, one maskis used for the plate line and one mask is used for the plate line viaat the end of the tree rows. Additionally, one mask must be used afterall layers are built to create the vias for a trunk portion 305. Incontrast, the second embodiment of a memory according to the presentinvention, shown in FIG. 9, requires 5N/2+1 masks because there are onlyhalf the number of tree branches.

FIG. 10 shows a cross-sectional view of an exemplary arrangement of athird embodiment of a 3-D 0T-FeRAM memory 1000 according to the presentinvention. In particular, FIG. 10 is a cross-sectional view showingdetails of the end of two rows of trees. Memory 1000 includes aplurality of memory cells 1001 that are each formed from a singleferroelectric capacitor and that are arranged in a memory-treestructure. Not all memory cells 1001 are indicated for simplification ofFIG. 10. Two memory trees 1002 a and 1002 b are arranged to be nearmirror images of each other so that a common voltage line 1003 can beshared for gain transistors 1004 a and 1004 b. Gain transistors 1004 andread transistors 1011 are used for converting a voltage on a treestructure into a current on a bit line for improved detectionsensitivity. Each memory tree 1002 includes a trunk portion 1005 that isformed from a conductive material, such as platinum, and a plurality ofbranch portions 1006 that are also formed from a conductive material,such as platinum. Memory cells 1001 are arranged above and below platelines 1007 so that plate lines 1007 connect to a branch portion 1006 oftwo different trees. The physical arrangement of memory cells 1001,branch portions 1006 and plate lines 1007 requires that only half of thenumber plate lines and plate-line drivers are needed in comparison tothe first embodiment of the present invention, shown in FIG. 3. Thephysical arrangement of memory cells 1001, branch portions 1006 andplate lines 1007 of the third embodiment of the present invention alsocauses data to be simultaneously placed on two different trees when aplate line 1007 is pulsed. Data from the two trees can be passed to thebit line in serial fashion (or separate bit lines can be used inparallel). Voltages may be written on the two different trees in aserial fashion.

Trunk portions 1005 are a series of vias between layers that are used toconnect to branch portions 1006 and can have a variety of possibleshapes. Cross-point access to a particular memory cell 1001 is through atrunk portion 1005 and branch portion 1006 and a plate-line 1007corresponding to the memory cell.

Multiple layers of storage cells are built on top of a single activesilicon layer 1010, which contains gain transistors 1004 a and 1004 b,read transistors 1011 a and 1011 b, write transistors 1012 a and 1012 b,and a 2-D array of plate-line drivers (not shown). Each tree 1002 a and1002 b has built-in sense gain by respectively connecting trunk portions1005 a and 1005 b to gates 1018 a and 1018 b of gain MOSFETs 1004 a and1004 b through conductive branches 1014 a and 1014 b. The potential ofthe memory tree is measured by turning on a read transistor 1011 duringa read operation and measuring the current flowing on bit line 1013 a,which connects to multiple rows of trees. A write transistor 1012 isused to place voltages on a memory tree during a write operation. Celldisturbance is managed by sequential access within a tree double row.That is, data is read and written by accessing each plate line in aserial manner until all memory cells in the double row of trees areaccessed.

In comparison to the first embodiment of a memory 300, the peripheralarea needed for plate-line drivers is reduced by a factor of two for thearrangement of the third embodiment of the present invention because asingle plate line connects to two branch portions. Additionally, thenumber of masks that is required is only 2N+2.

FIG. 11 shows a cross-sectional view of an exemplary arrangement of afourth embodiment of a 3-D 0T-FeRAM memory 1100 according to the presentinvention. In particular, FIG. 11 is a cross-sectional view showingdetails of the end of two rows of trees. Memory 1100 includes aplurality of memory cells 1101 that are each formed from a singleferroelectric capacitor and that are arranged in a memory-treestructure. Not all memory cells 1101 are indicated for simplification ofFIG. 11. Two memory trees 1102 a and 1102 b are arranged to be nearmirror images of each other so that a common voltage line 1103 can beshared for gain transistors 1104 a and 1104 b. Gain transistors 1104 andread transistors 1111 are used for converting a voltage on a treestructure into a current on a bit line for improved detectionsensitivity. Each memory tree 1102 includes a trunk portion 1105 that isformed from a conductive material, such as platinum, and a plurality ofbranch portions 1106 that are also formed from a conductive material,such as platinum. Memory cells 1101, branch portions, 1106 and platelines 1107 are arranged so that plate lines 1107 and inner branch 1106connect to two memory cells 1101. The physical arrangement of memorycells 1101, branch portions 1106 and plate lines 1107 requires that onlyhalf of the number plate lines and plate-line drivers are needed incomparison to the first embodiment of the present invention, shown inFIG. 3. The physical arrangement of memory cells 1101, branch portions1106 and plate lines 1107 of the fourth embodiment of a 3-D 0T-FeRAMmemory also causes data to be simultaneously placed on two differenttrees when a plate line 1107 is pulsed. Data from the two trees can bepassed to the bit line in serial fashion (or separate bit lines can beused in parallel). Voltages may be written on the two different trees ina serial fashion.

Trunk portions 1105 are a series of vias between layers that are used toconnect to branch portions 1106 and can have a variety of possibleshapes. Cross-point access to a particular memory cell 1101 is through atrunk portion 1105 and branch portion 1106 and a plate-line 1107corresponding to the memory cell.

Multiple layers of storage cells are built on top of a single activesilicon layer 1110, which contains gain transistors 1104 a and 1104 b,read transistors 1111 a and 1111 b, write transistors 1112 a and 1112 b,and a 2-D array of plate-line drivers (not shown). Each tree 1102 a and1102 b has built-in sense gain by respectively connecting trunk portions1105 a and 1105 b to gates 1118 a and 1118 b of gain MOSFETs 1104 a and1104 b through conductive branches 1114 a and 1114 b. The potential ofthe memory tree is measured by turning on a read transistor 1111 duringa read operation and measuring the current flowing on bit line 1113 a,which connects to multiple rows of trees. A write transistor 1112 isused to place voltages on a memory tree during a write operation. Celldisturbance is managed by sequential access within a tree double row.That is, data is read and written by accessing each plate line in aserial manner until all memory cells in the double row of trees areaccessed.

The number of masks that are required for the fourth embodiment of thepresent invention is only 3N/2+3. Consequently, for large N, the numberof masks is only about half the number of masks that are required forthe first embodiment of the present invention. Thus, for a given numberof masks, the fourth embodiment of the present invention has twice thenumber of capacitors and twice the number of bits for about half thecost per bit in comparison to the first embodiment of the presentinvention. For example, 16 layers of memory according to the fourthembodiment of the present invention can be created using only 27 masksteps.

While only four layers are shown in each of FIGS. 9-11, at least 16layers can be added to each embodiment of the present invention shown inFIGS. 9-11 by simply adding each additional layer to the top of a tree.Each additional layer accordingly increases the effective memorydensity. Also, the arrangement of memory cells shown in each of FIGS.9-11 applies equally well to either ferroelectric memory cells orhysteretic-resistor memory cells. Because a gain transistor is notneeded in the case of hysteretic-resistor memory cells, the siliconlayer electronics could be the same as is shown in FIG. 8.

FIG. 12 shows a cross-sectional view of an exemplary alternativearrangement 1200 of active components in the substrate below a 3-Dmemory according to the present invention. Alternative arrangement 1200has one fewer transistor than any of the arrangements shown in FIGS. 3and 9-11. In particular, unit transistors 1230 a and 1230 b respectivelyconnect tree portions 1205 a and 1205 b to conductive branch 1214 whenselected. Conductive branch 1214 is connected to gate 1214 a of a gaintransistor 1204. Gain transistor 1204 connects a common voltage line1203 to a read transistor 1211 during a read operation, which, in turn,is connected to a bit line 1213. Write transistor 1212 connects bit line1213 to conductive branch 1214 during a write operation.

Traditional 1T-1C ferroelectric memories typically have large cell areasthat are caused by difficulty in achieving a steep capacitor sidewallduring etching and by damage to the ferroelectric material caused duringetching. Consequently, planar capacitors are used having a largetrapezoidal shape, thereby increasing cell size. Additionally, etchingof ferroelectric material results in a loss of polarization. Furtherstill, a dedicated mask is used when a capacitor is etched. To avoidusing a dedicated mask for a capacitor in a cross-point memory, thepresent invention provides that the ferroelectric material can be etchedusing the same mask that is used when the branch portions and/or platelines are etched.

According to the present invention, capacitors are formed by firstdepositing one or more full films of conductors and a full film offerroelectric (perhaps also with a thin protective layer on top). Then,a branch is defined using a mask step and an etch step. This results ina multi-layered wire having a ferroelectric material as part of thewire. Subsequently, the “valleys” between memory cells 1201 are filledwith a dielectric material, such as silicon dioxide, and achemical-mechanical polishing (CMP) step is used for planarizing thewafer. At this point, the protective layer may be removed. Another setof conducting films, which will become plate lines, and full film offerroelectric material are then applied. Another protection layer can bedeposited on top of the ferroelectric material, and a second mask isused to define the first plate line. According to a first technique forforming ferroelectric memory cells according to the present invention,the second etch goes down all the way through the first ferroelectriclayer so that the first ferroelectric layer is etched a second time toform memory cells having a pillar-like structure, as depicted by memorycells 1301 in FIG. 13. Not all memory cells 1301 are indicated forsimplification of FIG. 13. The process is repeated so that eachferroelectric layer is etched twice to form memory cells having apillar-like structure without using a dedicated mask.

One very useful feature of cross-point memory is that it is notnecessary to form pillars, thereby avoiding the problems associated withtraditional 1T-1C memory. For example, FIG. 14 shows an arrangement of a3-D memory in which the ferroelectric material is etched only once intothe wire shape at the same time that the metal layer above it is etched,thereby further reducing the difficulties with etching. In FIG. 14,ferroelectric material 1401 and branch portions 1406 are formed as wiresusing one etching mask and ferroelectric material 1401 a and plate lineportions are formed as wires using one etching mask.

The concept of reducing the etching of the ferroelectric material layercan be extended, as shown by FIG. 15, which is the simplest arrangementhaving no ferroelectric etching at all. In FIG. 15, branch portions 1506are formed as wires using one etching mask. Subsequently, the “valleys”between branch portions 1506 are filled with a dielectric material suchas SiO₂ and a chemical-mechanical polishing (CMP) step is used forplanarizing the wafer. Then, a full film of ferroelectric material andmetal are deposited and the metal is etched to form the plate lineswithout significant etching of the ferroelectric layer. The arrangementshown in FIG. 15 avoids all problems with ferroelectric etching so thatscaling to nanometer dimensions is more readily attainable.

As described above, the output signal of a selected memory cell can bedetermined by comparing the output signal to the output of referencememory cells containing 0's and 1's. According to the present invention,each row of trees contains at least one tree in which all of the memorycells store 0's and at least one tree in which all of the memory cellsstore 1's. During a read phase, a selected plate line corresponding to aselected memory cell is brought high, thereby switching the polarizationof each memory cell along the selected plate line and changing thepotential of each respective tree structure along the selected plateline. When a memory cell contains a 1, for example, the voltage on thecorresponding tree structure changes accordingly. The conductance of again transistor corresponding to a tree structure changes depending onthe voltage on the tree structure. A current flows on a correspondingbit line that is a function on whether the capacitor of a memory cellcontained a 0 or a 1.

FIG. 16 is a schematic diagram illustrating an exemplary circuit 1600for detecting the output of a selected memory cell according to thepresent invention. To compensate for differences between capacitors indifferent memory layers caused by processing variations and differentfatigue and disturbance histories, the currents from multiple referencememory cells and from multiple reference trees are averaged using acurrent mirror. For example, FIG. 16 shows a current I_(X) that flows ona bit line BL_(N) as a result of the value stored on a selected memorycell (not shown). Current I₁ flowing on a bit line RBL₁ as a result of a1 being stored on a reference memory cell along the same plate line (notshown) and current I₀ flowing on a bit line RBL0 as a result of a 0being stored on a reference memory cell along the same plate line (notshown) is averaged by current mirror 1601. Current I_(X) is compared bycomparator circuit 1602 to averaged current (I₀+I₁)/2 and outputs eithera high or low signal on a SEN line depending on whether I_(X) is greateror less than (I₀+I₁)/2. While FIG. 16 shows the reference current outputfrom only two reference memory cells being averaged, it should beunderstood that each respective reference current could represent theaverage output current for a plurality of reference cells, therebyproducing a more precise reference value. The use of reference memorytrees can account for differences between capacitors along differentplate lines caused by processing variations and different fatigue anddisturbance histories.

The technique of the present invention is similar to the 2C/WL referencetechnique described by A. Sheikholeslami et al., “A Survey of CircuitInnovations in Ferroelectric Random-Access Memories,” Proceedings of theIEEE, Vol. 88, pp. 667-689 (2000), but differs by having two referencememory trees per row of trees rather than two capacitors per word linebecause of the unique 3-D architecture. The technique of the presentinvention solves many problems related to differences among capacitorsin different memory layers due to process variations and differencesbetween cells due to different fatigue and disturbance history forcapacitors along different plate lines.

FIGS. 17-19 depict an arrangement of a 3-D memory according to thepresent invention having tree structures that are placed only 2 F apart,in which F is the lithography half-pitch. This reduces the cell size bya factor of two and doubles the areal density. To achieve such closespacing, two bit lines must snake through the tree trunks in two levels,while making contact to two different trees in the same row of trees.The arrangement of active components in FIGS. 17-19 is similar to thearrangement of active components shown in FIGS. 3 and 9-12, and is notindicated to simplify FIGS. 17-19. Additionally, memory cells and branchportions are not indicated in FIGS. 17-19 to further simply FIGS. 17-19.

FIG. 17 shows the front two trees 1705 a and 1705 b of a pair of treerows. Tree trunk structure 1705, conductive branch structure 1714 andbit line structure 1713 associated with trees 1705 a and 1705 b shown bya block outline are physically in front of tree trunk, conductive branchand bit line structure shown by a diagonal cross hatching.

FIG. 18 shows the second two trees 1805 a and 1805 b in the pair of treerows, that is, the trees appearing behind trees 1705 a and 1705 b. Treetrunk structure 1805, conductive branch 1814 and bit line structure 1813associated with trees 1805 a and 1805 b shown by a block outline isphysically in front of tree trunk, conductive branch and bit linestructure shown by a diagonal cross hatching.

FIG. 19 shows the tree trunk and bit line structure associated with bothpairs of trees 1705 a and 1705 b, and 1805 a and 1805 b. Tree trunkstructure, conductive branch structure and bit line structure associatedwith the trees shown by a block outline is physically in front of treetrunk, conductive branch and bit line structure shown by a diagonalcross hatching. Many other possibilities exist.

Thus, the 3-D memory according to the present invention can potentiallyreplace FLASH memory for solid-state storage. Moreover, a cost analysisestimate suggests that the 3-D memory according to the present inventioncould even be a viable alternative to HDDs in the future. The very highperformance of ferroelectric memory of the invention relative to HDDscould also eliminate the need for large amounts of DRAM in computersystems.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced that are within the scope ofthe appended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the appended claims.

1-29. (canceled)
 30. A method for reading and erasing a memory device,the memory device including a plurality of bit lines formed on asubstrate and arranged substantially in a first plane and extendingsubstantially in a first direction, a plurality of layers, each layerhaving an array of ferroelectric capacitor memory cells, each layerbeing substantially parallel to the first plane, a plurality of treestructures arranged into a plurality of rows, at least one treestructure corresponding to each bit line and having a trunk portion andat least one branch portion, each branch portion of a tree structurecorresponding to at least one layer, the trunk portion of each treestructure extending from the corresponding bit line, each branch portionof a tree structure extending from the trunk portion of the treestructure, and each tree structure corresponding to a plurality oflayers, and a plurality of plate line groups, each plate line groupincluding a plurality of plate lines and corresponding to at least onelayer, each respective plate line group overlapping branch portions ofeach tree structure in at least one row of tree structures at aplurality of intersection regions, a ferroelectric capacitor memory cellbeing located at each intersection region in a layer, the methodcomprising: allowing each tree structure in at least one row toelectrically float near a first predetermined voltage; applying a secondpredetermined voltage V to a selected plate line; detecting a potentialof each tree structure in the at least one row; determining whether eachdetected potential corresponds to a 0 or a 1 for each memory cell at theintersections of the selected plate line and the tree structures in theat least one row; applying the first predetermined voltage to every treestructure in the at least one row; and applying the first predeterminedvoltage to the selected plate line.
 31. The method according to claim30, wherein the memory device further includes: a plurality of celllayer lines extending substantially in the first direction; a pluralityof cell column lines extending in a direction that is substantialperpendicular to the first direction and overlapping the plurality ofcell layer lines at a plurality of second intersection regions; and aplurality of plate-line driver transistors arranged in a two-dimensionalarray, each respective plate-line driver transistor corresponding to andbeing located at a respective second intersection region, and aconnection being formed between each plate line and the plate-linedriver corresponding to the plate line.
 32. The method according toclaim 30, wherein at least one memory cell has ferroelectric materialwith a pillar-like structure.
 33. The method device according to claim30, wherein a plurality of memory cells are connected by a ferroelectricwire.
 34. The method device according to claim 30, wherein a pluralityof memory cells are connected by a ferroelectric film.
 35. The methodaccording to claim 24, wherein the memory device further comprises: atleast one tree having a plurality of associated memory cells containinga first value; and at least one tree having a plurality of associatedmemory cells containing a second value that is different from the firstvalue; the method further comprising: outputting an average of an outputof each tree having a plurality of associated memory cells containingthe first value with an output of each tree having a plurality ofassociated memory cells containing the second value; and determining anoutput value of a selected memory cell of the memory device based on theaveraged output of each tree.
 36. A method for writing data to apreviously erased memory device, the memory device including a pluralityof bit lines formed on a substrate and arranged substantially in a firstplane and extending substantially in a first direction, a plurality oflayers, each layer having an array of ferroelectric capacitor memorycells, each layer being substantially parallel to the first plane, aplurality of tree structures arranged into a plurality of rows, at leastone tree structure corresponding to each bit line and having a trunkportion and at least one branch portion, each branch portion of a treestructure corresponding to at least one layer, the trunk portion of eachtree structure extending from the corresponding bit line, each branchportion of a tree structure extending from the trunk portion of the treestructure, and each tree structure corresponding to a plurality oflayers, and a plurality of plate line groups, each plate line groupincluding a plurality of plate lines and corresponding to at least onelayer, each respective plate line group overlapping branch portions ofeach tree structure in at least one row of tree structures at aplurality of intersection regions, a ferroelectric capacitor memory cellbeing located at each intersection region in a layer, the methodcomprising: applying a voltage V/3 to each tree structure in at leastone row of tree structures; applying a voltage 2V/3 to each plate linein the at least one row of tree structures; applying a voltage V to apredetermined number of selected tree structures in the at least one rowof tree structures; applying a voltage 0 to a selected plate line wherea data “1” will be written in a predetermined number of selected memorycells at the intersection of the first predetermined number of treestructures and the selected plate line; applying a voltage 2V/3 to theselected plate line; applying a voltage V/3 to the predetermined numberof selected tree structures in the at least one row of tree structures;applying a voltage 0 to each plate line in the at least one row of treestructures; and applying a voltage 0 to each tree structure in the atleast one row of tree structures.
 37. The method according to claim 36,wherein the memory device further includes: a plurality of cell layerlines extending substantially in the first direction; a plurality ofcell column lines extending in a direction that is substantialperpendicular to the first direction and overlapping the plurality ofcell layer lines at a plurality of second intersection regions; and aplurality of plate-line driver transistors arranged in a two-dimensionalarray, each respective plate-line driver transistor corresponding to andbeing located at a respective second intersection region, and aconnection being formed between each plate line and the plate-linedriver corresponding to the plate line.
 38. The method according toclaim 36, wherein at least one memory cell has ferroelectric materialwith a pillar-like structure.
 39. The method according to claim 36,wherein a plurality of memory cells are connected by a ferroelectricwire.
 40. The method according to claim 36, wherein a plurality ofmemory cells are connected by a ferroelectric film.
 41. The methodaccording to claim 36, wherein the memory device further comprises: atleast one tree having a plurality of associated memory cells containinga first value; and at least one tree having a plurality of associatedmemory cells containing a second value that is different from the firstvalue; the method further comprising: outputting an average of an outputof each tree having a plurality of associated memory cells containingthe first value with an output of each tree having a plurality ofassociated memory cells containing the second value; and determining anoutput value of a selected memory cell of the memory device based on theaveraged output of each tree.
 42. A method for reading and erasing amemory device, the memory device including a plurality of bit linesformed on a substrate and arranged substantially in a first plane andextending substantially in a first direction, a three-dimensional memoryhaving a plurality of layers and a plurality of tree structures, eachlayer having a plurality of memory cells and the tree structures beingarranged in a plurality of rows, each tree structure having a trunkportion and at least one branch portion, each branch portion of a treestructure corresponding to at least one layer, and each branch portionof a tree structure extending from the trunk portion of the treestructure, a plurality of plate line groups, each plate line groupincluding a plurality of plate lines and corresponding to at least oneeach layer of the three-dimensional memory, each respective plate linegroup overlapping the branch portion of each tree structure in at leastone row of tree structures at a plurality of intersection regions, amemory cell being located at each intersection region in a layer, and aplurality of plate-line driver transistors formed on the substrate andarranged in a two-dimensional array, each plate-line driver transistorcorresponding to a plate line, the method comprising: allowing each treestructure in at least one row to electrically float near a firstpredetermined voltage; applying a second predetermined voltage V to aselected plate line; detecting a potential of each tree structure in theat least one row; determining whether each detected potentialcorresponds to a 0 or a 1 for each memory cell at the intersections ofthe selected plate line and the tree structures in the at least one row;applying the first predetermined voltage to every tree structure in theat least one row; and applying the first predetermined voltage to theselected plate line.
 43. The method according to claim 42, wherein atleast one memory cell is a ferroelectric memory cell.
 44. The methodaccording to claim 43, wherein at least one memory cell hasferroelectric material with a pillar-like structure.
 45. The methodaccording to claim 43, wherein a plurality of memory cells are connectedby a ferroelectric wire.
 46. The method according to claim 43, wherein aplurality of memory cells are connected by a ferroelectric film.
 47. Themethod according to claim 42, wherein the memory device furthercomprises: at least one tree having a plurality of associated memorycells containing a first value; and at least one tree having a pluralityof associated memory cells containing a second value that is differentfrom the first value; the method further comprising outputting anaverage of an output of each tree having a plurality of associatedmemory cells containing the first value with an output of each treehaving a plurality of associated memory cells containing the secondvalue; and determining an output value of a selected memory cell of thememory device based on the averaged output of each tree.
 48. The methodaccording to claim 42, wherein the memory device further comprises: aplurality of cell layer lines extending substantially in the firstdirection; and a plurality of cell column lines extending in a directionthat is substantial perpendicular to the first direction and overlappingthe plurality of cell layer lines at a plurality of second intersectionregions, and wherein each respective plate-line driver transistorcorresponding to and being located at a respective second intersectionregion, and a connection being formed between each plate line and theplate-line driver corresponding to the plate line.
 49. A method forwriting data to a previously erased memory device, the memory deviceincluding a plurality of bit lines formed on a substrate and arrangedsubstantially in a first plane and extending substantially in a firstdirection, a three-dimensional memory having a plurality of layers and aplurality of tree structures, each layer having a plurality of memorycells and the tree structures being arranged in a plurality of rows,each tree structure having a trunk portion and at least one branchportion, each branch portion of a tree structure corresponding to atleast one layer, and each branch portion of a tree structure extendingfrom the trunk portion of the tree structure, a plurality of plate linegroups, each plate line group including a plurality of plate lines andcorresponding to at least one layer of the three-dimensional memory,each respective plate line group overlapping branch portions of eachtree structure in at least one row of tree structures at a plurality ofintersection regions, a memory cell being located at each intersectionregion in a layer, and a plurality of plate-line driver transistorsformed on the substrate and arranged in a two-dimensional array, eachplate-line driver transistor corresponding to a plate line, the methodcomprising: applying a voltage V/3 to each tree structure in at leastone row of tree structures; applying a voltage 2V/3 to each plate linein the at least one row of tree structures; applying a voltage V to apredetermined number of selected tree structures in the at least one rowof tree structures; applying a voltage 0 to a selected plate line wherea data “1” will be written in a predetermined number of selected memorycells at the intersection of the first predetermined number of treestructures and the selected plate line; applying a voltage 2V/3 to theselected plate line; applying a voltage V/3 to the predetermined numberof selected tree structures in the at least one row of tree structures;applying a voltage 0 to each plate line in the at least one row of treestructures; and applying a voltage 0 to each tree structure in the atleast one row of tree structures.
 50. The method according to claim 49,wherein at least one memory cell is a ferroelectric memory cell.
 51. Themethod according to claim 50, wherein at least one memory cell hasferroelectric material with a pillar-like structure.
 52. The methodaccording to claim 50, wherein a plurality of memory cells are connectedby a ferroelectric wire.
 53. (canceled)
 54. The method according toclaim 49, wherein the memory device further comprises: at least one treehaving a plurality of associated memory cells containing a first value;and at least one tree having a plurality of associated memory cellscontaining a second value that is different from the first value; themethod further comprising: outputting an average of an output of eachtree having a plurality of associated memory cells containing the firstvalue with an output of each tree having a plurality of associatedmemory cells containing the second value; and determining an outputvalue of a selected memory cell of the memory device based on theaveraged output of each tree.
 55. The method according to claim 49,wherein the memory device further comprises: a plurality of cell layerlines extending substantially in the first direction; and a plurality ofcell column lines extending in a direction that is substantialperpendicular to the first direction and overlapping the plurality ofcell layer lines at a plurality of second intersection regions, andwherein each respective plate line driver transistor corresponding toand being located at a respective second intersection region, and aconnection being formed between each plate line and the plate linedriver corresponding to the plate line.